Various means are currently available to separate the components of a multicomponent fluid mixture. If the densities of the components differ sufficiently, the effects of gravity over time may be adequate to separate the components. Depending on the quantities of the components involved, a centrifuge may be used to more rapidly separate components with different densities. Alternatively, distillation may be used to separate components with different boiling points.
Some fluid mixtures comprise components which have similar boiling points, and in such cases, separation by distillation may be a difficult and an inefficient means to separate these components. Too many contaminants, e. g., unwanted components, also may evaporate along with (or fail to evaporate from) the desired component(s), or the separation may require high energy expenditures due to the recycling through the distillation process that may be necessary to attain a desired degree of separation or purity.
In view of these and other deficiencies of these aforementioned processes, adsorption often has been preferred as a process for separating the components from a multicomponent fluid mixture to obtain relatively pure products.
The efficiency of an adsorption process may be partially dependent upon the amount of the surface area of the adsorbent solids which is available for contact with a fluid mixture. The surface area available may be more than just the superficial, external surface of the solids. Suitable solids also may have internal spaces. Such internal spaces may comprise pores, channels, or holes in the surface of the solids and may run throughout the solids, much as in sponges. Thus, the fluid contacts not only the superficial surface, but penetrates into the solids. Sieve chambers increase the contact surface between the fluid and the solids in an adsorption process by concentrating them in a confined space. Such structures often are described as molecular sieves, and the volumetric amount of components that may be adsorbed by a molecular sieve is termed the molecular sieve capacity.
In an adsorption process, separation of the fluid components may be accomplished because the material may have a physical attraction for one or more of the components of the mixture in preference to other components of the mixture. Although all of the components of a mixture may be attracted in varying degrees to the material, there is a preference engineered into the process, such that predominantly the desired component(s) may be attracted and remain with the material in preference over all others. Therefore, even if less preferred components of a mixture initially come into contact with a portion of the material, because of the stronger attraction of the material for the desired component(s) of the mixture, the less preferred component(s) may be displaced from the material by the desired, and more strongly preferred, component(s). Although the fluid mixture entering a sieve chamber might be composed of multiple components, the fluid mixture initially leaving the vessel would be composed largely of the components which had been less preferentially adsorbed into the material.
In adsorption processes using adsorbent solids, separation occurs for a period of time, but eventually all the available surface sites on and in the solids are taken up by the desired component(s) or are blocked by concentrations of unwanted components. At that point, little significant additional adsorption of component(s) from the mixture is likely to occur, and the fluid mixture which might be withdrawn from the chamber may be insignificantly changed by further exposure to the solids. The adsorption step of the process is thus ended, and the component(s) which have been adsorbed by the solids must be removed from the solids, so as to effect separation and permit reuse of the solids.
A suitable adsorption apparatus or system might first permit adsorption of a product comprising the desired component(s) by the solids and later treat the solids to cause them to release the product and permit recovery of this product. Such an adsorption apparatus or system might comprise a “moving-bed” which permits movement of a tray or bed of the solids through a chamber, such that at different locations, the solid is subjected to different steps of an adsorption process, e.g., adsorption, purification, and desorption. These steps are defined in greater detail below. Nevertheless, moving the solids through an adsorption apparatus may be difficult and involve complex machinery to move trays or beds. It also may result in loss of the solids by attrition. To avoid these problems, some adsorption apparatus and systems have been designed to “simulate” moving the tray(s) or bed(s) to the locations, e.g., zones, of different steps of an adsorption process. Simulation of the movement of the tray(s) or bed(s) may be accomplished by use of a system of conduits which permits directing and redirecting the streams of fluids into the chamber at different zones at different times. As these stream changes occur, the solids are employed in different steps in an adsorption process as though the solids were moving through the chamber.
The different zones within an adsorption apparatus or system are defined by the particular step of the adsorption process performed within each zone, e.g., (1) an adsorption step in the adsorption zone; (2) a purification step in the purification zone; (3) a desorption step in the desorption zone. A more detailed explanation of the zones of the adsorption process follows.
Adsorption Zone: when a multicomponent fluid feedstream, such as a feedstream comprising the C8 aromatics orthoxylene (OX), metaxylene (MX), paraxylene (PX), and ethylbenzene (EB), is fed into the adsorption apparatus or system, the portion of the apparatus or system into which the feedstream is being fed is termed an “adsorption zone.” In the adsorption zone, the fluid comes into contact with the adsorbent material, and the desired component(s) are adsorbed by the adsorbent material. As noted above, other components may also be adsorbed, but preferably to a lesser extent. This preferential adsorption may be achieved by the selection of an adsorbent material, e.g., adsorbent solids, which have a preference for adsorbing the desired component(s) from the multicomponent feedstream. Although only the desired component(s) may have been adsorbed by the solids, other less preferentially adsorbed components of the fluid mixture may still remain in void spaces between the solids and possibly, in the pores, channels, or holes within the solids. These unwanted components preferably are removed from the solids before the desired component(s) are recovered from the solids, so that they are not recovered along with the product.
Purification Zone: after adsorption, the next step is to purify the fluid and adsorbent material in the chamber. In this step, the tray(s) or bed(s) may be moved or flow within the conduits may be changed, so that the multicomponent feedstream may no longer be fed into the adsorption zone. Although the tray(s) or bed(s) have not physically moved, the material may now be described as being in a “purification zone” because a fluid stream, e. g., a purification stream, is fed into the adsorbent material to flush the unwanted components from the adsorbent material, e.g., from within and from the interstitial areas between the solids. Thus, a fluid comprising unwanted components, i.e., raffinate, is flushed from the purification zone by substituting a fluid comprising the desired component(s) or other component(s) deemed to be more acceptable for the unwanted components. The unwanted components may be withdrawn in a raffinate stream. Because an objective of the adsorption process may be to separate the product comprising the desired component(s) from other components which may have nearly the same boiling point or density as the desired component(s), purification may displace, unwanted components and substitute another fluid which can be more readily separated by other means, e.g., distilled.
Desorption Zone. after the solids have been subjected to the purification stream, the stream in the conduit(s) may again be changed to introduce a desorbent steam into the chamber to release the product. The desorbent stream contains desorbent which is more preferentially adsorbed by the solids than the product comprising the desired component(s). The desorbent chosen will depend in part upon the desired component(s), the adsorbent materials, and the ease with which the desorbent can be separated from the product. Once the desorbent stream has been introduced to the chamber, the product may be withdrawn from the chamber.
Each and every step and zone might be present somewhere in an adsorption apparatus or system if simultaneous operations are conducted. Nevertheless, the steps may be performed successively or staggered over time. Further, in some adsorption processes, the unwanted components may be adsorbed, and the product comprising the desired component(s) allowed to pass through the adsorption apparatus or system. Therefore, the terms raffinate and extract are relative and may depend upon the particular nature of the components being separated, the preference of the solids, and the nature of the apparatus or system. Although in embodiments the present invention will be discussed primarily in terms of apparatus and systems in which the product is adsorbed by the solids, the invention is not limited to such configurations.
An apparatus suitable for accomplishing the adsorption process of this invention is a simulated moving-bed adsorption apparatus. A commercial embodiment of a simulated moving-bed adsorption apparatus is used in the well-known Parex Process, which is used to separate C8 aromatic isomers and provide a more highly pure paraxylene (PX) from a less highly pure mixture. See U.S. Pat. Nos. 3,201,491; 3,761,533; and 4,029,717.
Typically, such an adsorption apparatus is contained in a vertical chamber. Such a chamber may be packed with adsorbent solids, possibly in trays or beds stacked within the chamber. More than one type of solid also might be used. The chamber also may have the capability to perform each of the above-described steps simultaneously within different locations, e.g., zones, in the chamber. Thus, the composition of the fluid in the chamber may vary between zones although there may be no structures completely separating these zones. This may be achieved by the use of a serially and circularly interconnected matrix of fluid communication conduits including associated means, such as valves and pumps, which permit streams to be directed and redirected into different zones of the chamber and to change the direction of these streams through the solids within the different zones of the chamber. The different zones within the chamber may have constantly shifting boundaries, as the process is performed.
The cyclic advancement of the streams through the solids in a simulated moving-bed adsorption apparatus may be accomplished by utilizing a manifold arrangement to cause the fluid to flow in a counter current manner with respect to the solids. The valves in the manifold may be operated in a sequential manner to effect the shifting of the streams in the same direction as overall fluid flow throughout the adsorbent solids. In this regard see U.S. Pat. No. 3,706,812. Another means for producing a countercurrent flow in the solid adsorbent is a rotating disc valve by which the streams, e.g., feed, extract, desorbent, and raffinate, and line flush, are advanced cyclically in the same direction through the adsorbent solids. Both U.S. Pat. Nos. 3,040,777 and 3,422,848 disclose suitable rotary valves. Both suitable manifold arrangements and disc valves are known in the art. More recently, a system has been described using dual rotary valves (U.S. application Ser. No. 61/116097, filed 19 Nov. 2008).
Normally there are at least four streams (feed, desorbent, extract, and raffinate) employed in the procedure. The location at which the feed and desorbent streams enter the chamber and the extract and raffinate streams leave the chamber are simultaneously shift in the same direction at set intervals. Each shift in location of these transfer points delivers or removes liquid from a different bed within the chamber. In many instances, one zone may contain a larger quantity of adsorbent material than other zones. Moreover, zones other than those discussed above may also be present. For example, in some configurations, a buffer zone between the adsorption zone and the desorption zone may be present and contain a small amount of adsorbent material relative to the zones surrounding it. Further, if a desorbent is used that can easily desorb extract from the adsorbent material, only a small amount of the material need be present in the desorption zone in comparison to the other zones. In addition, the adsorbent need not be located in a single chamber, but may be located in multiple chamber or a series of chambers.
Introducing and withdrawing fluids to the beds may comprise a plurality of fluid communication conduits, and the same fluid communication conduit may be used in a first instance to input a feedstream into the apparatus or system and later to withdraw an extract stream. This can result in reduced product purity due to contamination of the withdrawn product. Fluid communication conduits may contain unwanted components, such as residue remaining in the conduit from earlier additions or withdrawals of streams. This problem may be overcome by employing separate conduits for each stream or by removing such residue from the conduits by flushing them with a medium which would not effect product purity as adversely as would a unwanted component remaining in the fluid communication conduit. A preferred flushing medium has been the product or the desorbent, which might be more readily separated downstream of the chamber than would the residue. See U.S. Pat. No. 4,031,156. Nevertheless, flushing conduits with the product reduces the output of the adsorption process.
A standard Parex unit for separating paraxylene (PX) from metaxylene (MX), and orthoxylene (OX) has a single feed to a single rotary valve or parallel rotary valves. The rotary valve directs the feed to a bed line, which is somewhere between the extract (which may comprise, by way of example, 99.7% PX and desorbent) and the raffinate (PX-depleted xylenes and desorbent) withdrawal points. Since the UOP Parex process is a simulated moving bed process, the bed lines are shared with all of the feed and product streams, and therefore the bed lines must be flushed between the feed injection point and the extract withdrawal point in order to prevent contamination of the product.
A standard UOP parex unit has a primary flush which removes the majority of contaminants and a secondary flush which removes trace impurities just before the extract point. Because the standard UOP Parex unit has a single feed, various streams of different compositions are typically blended together and fed to a single point in the Parex process.
However, as indicated in U.S. Pat. No. 5,750,820, and also U.S. Pat. No. 7,396,973, it is better to segregate feeds which are of substantially different composition, such as concentrated paraxylene from a selective toluene disproportionation unit (generally 85-90% paraxylene) and equilibrium xylenes (generally about 23% paraxylene) from a powerformer, isomerization unit or transalkylation unit. This can be done by using the primary line flush as a second feed point for the paraxylene concentrate and using the secondary flush as the sole flushing stream. Having only a single flush does result in a slight compromise in the separation process, but the compromise typically is far outweighed by the benefit of optimizing the feed location of the paraxylene concentrate as far as net purity in the final product.
However, there is a problem with the above configuration in that the standard Parex unit has the secondary flush located close to the extract withdrawal point in order to minimize contaminants that are withdrawn with the extract. When the secondary flush is very close to the extract withdrawal point, the concentrated paraxylene that is being flushed from the bed line will be too close to the extract withdrawal point and the highest separation of the feed will not be realized. This has heretofore not been recognized.
The present inventors have surprisingly discovered that the feed location of both the concentrated paraxylene in the primary flush and also the location of the secondary flush can be modified to realize the full benefit of the feed configuration in U.S. Pat. No. 5,750,820. By moving the secondary flush further away from the extract, the material flushed from the bed line will be injected at a more efficient location. In embodiments, improvements associated with less desorbent recirculation can be realized.